For the purpose of profitable reutilization, waste rubber or rubber products need to be ground to a monodisperse powder of the minimum possible grain size. This powder can then be successfully used as a high-grade filler or additive for the production of new rubber articles. Firstly, this kind of rubber recycling helps to reduce the amount of waste, and secondly, it contributes to returning a valuable raw material into the production cycle.
The processes and apparatuses that are known so far for pulverizing such polymers have not brought satisfactory results. Published German patent application 2,145,728 (corresponds to U.S. patent application Ser. No. 071,896, filed Sep. 14, 1970), for example, describes a process for the production of reusable rubber or caoutchouc from waste rubber, which brings the waste rubber into contact with a cryogenic medium, thus cooling it down to a temperature of below -40.degree. C. The resulting brittleness of the material facilitates mechanical crushing to fine powder without the individual rubber grains agglomerating. The disadvantage of this process and other processes that are based on making the material brittle by means of cooling is the very high energy requirement, which renders the powdered rubber too expensive a raw material for new rubber articles.
In addition, published German patent application 2,315,587 discloses an apparatus for pulverizing rubber material by introducing the rubber material between a rotor of elliptical cross section and the surrounding housing. By rotating the rotor in relation to the housing, shearing and frictional forces are exerted on the rubber material and break it up in a continuous flow into particles of the required size. According to a preferred embodiment, the rubber material allows particularly effective pulverization if it is fed into the housing under pressure or if it is forced into the housing while the external diameter of the inner housing walls is changed continuously.
The experience gained with this apparatus or similar ones was not satisfactory. On the one hand, a line with variable inner housing diameter operating economically with this process was too expensive from a technical point of view, and on the other hand, the powdered rubber produced in this way did not show the desired fineness and widely monodisperse distribution with regard to the grain diameter. This is mainly due to an insufficient heat removal from the ground stock, thus causing already finely ground rubber particles to agglomerate to larger grains. Furthermore, the screening of the powdered rubber, which is necessary for its reutilization, increased the cost of the powder considerably.